Fix static checker dogfooding warning - make un-mutated field readonly
Fix static checker dogfooding warning - make un-mutated field readonly

Microsoft/CodeContracts,Microsoft/CodeContracts,Microsoft/CodeContracts,Microsoft/CodeContracts,Microsoft/CodeContracts,Microsoft/CodeContracts,ndykman/CodeContracts,Microsoft/CodeContracts,ndykman/CodeContracts,ndykman/CodeContracts,ndykman/CodeContracts,ndykman/CodeContracts,ndykman/CodeContracts,ndykman/CodeContracts